1. Field of the Invention
This invention relates to the interconnection of microelectronic chips. The interconnections may be between chips on a multi-chip module, between several multi-chip modules, or even between distant points on a larger chip. More particularly, it pertains to the use of optical wires bonded on those chips to interconnect the chips. The interconnections by means of optical fibers are made to substitute for the electrical wire interconnections. Each optical wire terminates at a small laser chiplet on one end and a photodetector chiplet on the other end. Each chiplet is flip-chip mounted onto the larger electronic chips and each contains a vertically coupled laser or photodetector and solder bumps on one face and a deflecting mirror and a V-groove (into which one end of the optical fiber is inserted) on the opposite face.
2. Description of the Related Art
In a high speed multichip module (MCM) environment, chip-to-chip connections are usually made using bond wires, with microstrip lines on the MCM substrate used to interconnect chips that are farther apart.
Presently, electrical bond wires are used to interconnect microchips. Using the electrical wires has serious drawbacks. The electrical wires are sensitive to electromagnetic interference and themselves create such interference which poses especially serious problems for distribution of timing signals. The electrical wires must be located at the edges of chips. Signal attenuation and phase delay depend upon the length of the electrical wires. Thus, depending on the lengths of the electrical wires and their locations in the module, it may be difficult to achieve equal attenuation and/or equal signal phase delay among multiple wires, if needed.
In addition, in many cases signal bandwidths of several Gigahertz are desirable but cannot be achieved if electrical wires are used because electrical bond wires act as open antennae at high frequencies and introduce noise coupling among the wires. For example, bond wires of 500 micrometers in length and 1 mil (0.001 inch) diameter carrying 10 milliamperes of current will produce appreciable (100 millivolts or more) coupling or cross-talk at 10 Gigahertz even when they are spaced several pitch distances apart, a typical pitch being 100 to 150 micrometers. This effect will substantially limit the maximum speed of a typical MCM module having hundreds of bond wires from several chips. The cross-talk is even more severe when the chips are located farther apart and require longer bond wires.
Therefore, there is a need to have interconnects between microchips which:
(a) are insensitive to electromagnetic interference;
(b) need not be located at the edges of a chip but rather can be placed for optimal utility to the circuit function;
(c) can be given the same or other pre-specified lengths regardless of the placement in the module; and
(d) are capable of signal bandwidths up to 20 Gigahertz without causing the cross-talk problem.
Optical bond-wire interconnections satisfy all these requirements. Previously, optoelectronic devices such as vertical-cavity lasers and photodetectors have been bonded onto microelectronic chips to provide free-space optical interconnections and the results were reported, for instance, by D. A. Louderback, et. al., in xe2x80x9cModulation and Free-Space Link Characteristics of Monolithically Integrated Vertical-Cavity Lasers and Photodetectors with Microlensesxe2x80x9d, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 2 (1999).
However, for such free-space interconnections, the optoelectronic devices must be installed in a way that they face one another. Moreover, their relative locations must be precisely controlled to ensure optical alignment. As a result, in free-space optical interconnections, the optoelectronic devices must be located on different multi-chip modules that are held in immediately adjacent slots of a rack.
With optical interconnect wires bonded directly onto microelectronic chips there is almost no constraint on the locations of the chips to be interconnected. The chips may reside on the same multi-chip module or may be disposed many meters apart. These chips can even be members of different instruments or computation units; however, if the optical-fiber bond-wire is subject to movement, then some mechanical means is preferably provided to relieve the optical fiber and chiplets from excessive strain.
In the prior art, optical fibers are typically coupled to optoelectronic devices using an accompanying sub-mount, such as a machined piece of a metal, or ceramic, or a V-grooved silicon substrate, when both the optical fiber and the optoelectronic device chip are mounted on the sub-mount. Directly attaching and optically aligning an optical fiber to an optoelectronic chip would be most beneficial.
There exists no known prior art for fiber-based optical interconnects bonded directly onto microelectronic chips. Yet, as discussed above, the need for such is acute.
For the foregoing reasons, there is a necessity for optical bond-wire interconnections. The present invention discloses such interconnections.
The present invention is directed to an optical bond-wire interconnect and to a method of manufacturing of the interconnect. It can be used instead of electrical bond-wires but can be much longer than the electrical bond-wires. For instance, length of an electrical wire typically does not exceed maximum length of 1 centimeter and is usually shorter. An optical bond-wire can reach lengths of the order of hundreds of meters.
Each optical bond-wire comprises a segment of optical fiber that is attached at its two ends by means of terminations to the microelectronic chip or chips. The two terminations of the optical bond-wire are a laser chiplet on one end of the optical bond-wire and a photodetector chiplet on the other end. Each chiplet can be as small as 250 by 250 micrometers and is connected to two electrical linesxe2x80x94one line is the signal to be sent via the interconnect and the other line is a return or ground.